2 Composition of Cast Iron.
Most of the impurities disappear in the blast furnace, but
carbon is absorbed from the coke fuel, and the presence of this
carbon, mechanically mixed in the form of graphite, makes the
iron more liquid when molten, but at the same time produces
weakness in the casting. ,'Th.ere is never more than five per cent,
of uncornbined carbon, while in the white iron there is almost
none, it being chemically combined, and then actually increases
the strength of the iron.
Table showing chemical composition of the three principal
varieties of pig iron, in percentages :
Grey. 90*24. ....
)*3 79 *ii
"17 48 17 . 6
White. 89-86 2-46 87
Carbon (combined) ...
2*64 ..... vo6 .....
Graphite (uncornbined) ..... Silicon ................ . ........
> . : 1*14. ....
: -81 .....
^ The Cupola.The pig iron is re-melted in the foundry in a
kind of small blast furnace called a Cupola. The cupola is re-lit
very day (and is therefore not so economical as a blast furnace,
where the fire is never allowed to die out),* but this cannot be
avoided on account of the intermittent demand made upon it.
Fig. i is such a cupola, where the pigs and coke are raised by
the lift H L, hydraulic or otherwise, together with the man, who,
after breaking each pig in three, puts them all in at the door D,
charging as follows:First, 7 cwts. of coke, next i ton of iron;
then^ alternately, 2 cwts. of coke and i ton of iron, until the
cupola is filled to D. The blast enters at B, and the mouth M is
stopped with luting clay. When all -the iror* is meked M is tapped,
and the metal taken away in ladks to the moulds.
During re-melting the iron is again apt to absorb impurities
from the fuel, such as oxides and silicates, the latter especially
producing more brittle material, and rendering the iron cold-short,
that is, easily snapped when cold. Formerly, re-melting was
believed to be an improvement, and founders were advised to
* One blast furnace in the North of England, known to the writer, burned
for over twelve years incessantly, and was then only blown out for repairs.